Origin: Australian/American colloquialism.
Based on Happy Days character.... though evolving over the years...

Used for both Male or Female

Defined: Someone who thinks they are a bit too cool... Generally the person who dresses a bit over the top and/or always trying to "one-up" someone else’s comments, whilst tending to overdo everything they try to do....

Example:

In a Night-club where everyone is in cargo pants and shirts... “chachi” rocks up with the denim jacket and greased back hair, and sun glasses on his head at 2am...thinking he’s a little too good for everyone else at the club.

A Chachi is a heterosexual male who could be charectorized as a metrosexual with a white-trash-won-the-lotto clothing style. They wear all the major designer labels but clash horribly as they mix styles at random. They also sport strange hairstyles and mullet remixes and tend to wear blush and lipgloss. They can be found singing along to hardcore rap, boybands, or dancing to techno tunes in the local clubs. Fortunatly this awful metro sub genre is for the time being found only in Europe and very prodomninatly in Switzerland. If you are ever in Zurich you will be sure to see several hanging out at the local McDonalds before clubbing.

Or:
Anything trashy, tacky or tastless

You: " Hey that Chachi just checked you out.."

Me: "Wow I think that kid wears more makeup than a 14 year old hooker. Or is that a 14 year old hooker?....hmm... nope it's a Chachi"